1. Field of the Invention
The present invention relates to a method for manufacturing a photoelectric conversion device using a single crystal semiconductor.
2. Description of the Related Art
The industrial growth has been boosting energy consumption worldwide. It is said that carbon dioxide is produced due to consumption of oil, coal, natural gas, and the like which are mainly used as energy resources, which has been a factor of drastic global warming. Therefore, photovoltaic power generation is being spread for alternative energy in recent years.
For photovoltaic power generation, although solar heat may be utilized, mainly employed is a method of converting light energy into electric energy with use of the photoelectric characteristic of a semiconductor. Devices for converting light energy into electric energy are generally called photoelectric conversion devices (also called photoelectromotive devices, solar cells, or the like).
Photoelectric conversion devices are already marketed and the production thereof has been expanding year by year with government supports for solar cells around the world. For example, the production of solar cells around the world in 2006 is 2521 MW, which has increased by more than 40% per annum. Here, above all, photoelectric conversion devices including crystalline semiconductors have become popular worldwide, and a large part of the production is occupied by the devices including single crystal silicon substrates or polycrystalline silicon substrates.
A variety of structures of photoelectric conversion devices have been disclosed. In addition to a typical structure in which an n-type or a p-type diffusion layer is formed in a single crystal silicon substrate or a polycrystalline silicon substrate, known is a structure in which a unit cell including a single crystal semiconductor or a polycrystalline semiconductor and a unit cell including an amorphous semiconductor or a microcrystalline semiconductor are combined (for example, see Patent Document 1: Examined Patent Application Publication No. H6-044638). Note that also in that case, a single crystal silicon substrate or a polycrystalline silicon substrate is used.
With the increase in production of photoelectric conversion devices, short of supply and rise of cost of raw material silicon which is the material of single crystal silicon or polycrystalline silicon, have become significant problems of the industry. Although major silicon suppliers in the world have already tried to increase capability of silicon production, the increase in demand outweighs the capability and the short of supply does not seem to be solved for some time.
In the case of using crystalline silicon, a thickness of about 10 μm is enough for the thickness of a silicon thin film. However, a single crystal silicon wafer is generally manufactured with a thickness of from about 600 μm to about 800 μm, whereas a polycrystalline silicon wafer is generally manufactured with a thickness of from about 200 μm to about 350 μm. That is to say, the thickness of a single crystal silicon substrate or a polycrystalline silicon substrate is several tens of times as large as a thickness required to form a photoelectric conversion device and the raw material Si is not used effectively. In view of this problem, it can be said that there is room for improvement in current photoelectric conversion devices.
In recent years, development has been actively conducted on an SOI (silicon on insulator) structure that includes a single crystal silicon thin film formed over a substrate having an insulating surface. Although an SOI substrate is expensive, the cost of the SOI substrate can be reduced by using an inexpensive substrate such as a glass substrate as a supporting substrate in comparison with the case of using a single crystal silicon substrate as a supporting substrate. Further, the consumption of silicon, which is a raw material, can be reduced. For such a technique, a method for manufacturing an SOI substrate in which a single crystal silicon thin film is fixed to a glass substrate is disclosed (for example, see Patent Document 2: Japanese Published Patent Application No. H11-097379).